Central locking systems of the type in which the vehicle has at least one electrically operated door lock and which utilize a remote control to operate that door lock are known, e.g. from U.S. Pat. No. 4,596,985 and German Patent 32 44 049.
The remote control unit can include a transmitter functioning as a code key and a receiver functioning as a code lock with the coding unit in the transmitter coding a transmitted signal which is subjected, at the receiver, to decoding by a complementary decoding unit generating an actuating signal for the control electronics of the vehicle door lock or locks of the hood, trunk or other compartments which may be provided on the vehicle.
Remote operating systems can generally be referred to as wireless and contactless systems since the transmission of the signal between the transmitter and receiver takes place without a direct interconnection of the two, utilizing waves propagated through the space between them. The code key and the code lock are information-transmission and electronic analogs for bitting of the key and the lock cylinder of mechanical door locks.
Since it is desirable that the central locking system respond to the transmitter, i.e. the lock be actuated, only when the transmitter sends a particular coded signal generated by the coding device, the decoding device of the receiver must have a matching stored code which can correspond to the coded transmission signal and which upon decoding, operates the lock. When there is this match, the control electronics of the lock will receive from the decoding unit an actuating signal which can be, for example, a pulse or a pulse train or pulse sequence. When this condition is not fulfilled, i.e. the coded signal from the transmitter is not recognized at the decoder, the control electronics remains unactivated.
Upon decoding of a received transmitted signal which does not match the stored code at the decoder, other functions can be initiated, for example, a time-limited inactivation of the entire central locking system can be enforced, disabling the receiver and the control electronics for a certain period of time.
Central locking systems of this type have been described in the aforementioned U.S. Pat. No. 4,596,985 as well as German Patent 32 44 049. In these systems, transmitters and receivers are provided which operate with electromagnetic waves which can span the entire spectrum of electromagnetic waves including radio frequency and light frequencies. The coding unit and the oscillator at the transmitter on the one hand and the detector and the decoding unit at the receiver on the other, of course, must communicate with the same mode of electromagnetic transmission and with the same frequency.
It is important that the coding unit and the detecting unit be matched regardless of the frequency or mode, i.e. radio frequency or light frequency of the high-frequency carrier of the associated oscillator and detector.
The term "high-frequency carrier" is not intended here to refer to waves of a particular frequency range but rather is intended to refer to a carrier which can be modulated with the comparatively lower frequency code.
The limitations on the carrier frequencies of a particular oscillator and detector path have, however, posed problems in the use of central locking systems for automotive vehicles both in terms of the physical restrictions and even in terms of legal restrictions with respect to the band width and frequencies used of radio frequencies, for example, and the detrimental effects of some type of light frequencies. The band width of radio frequencies must be increased with increasing complexity of the codes transmitted for various purposes and with the increasing transmission speeds which may be required. The band width requirements differ from amplitude-modulation systems to frequency-modulation systems or pulse-duration modulators.
The life effects of use of oscillators operating at light frequencies can be of a thermal and/or quantum mechanical nature. For example, the maximum frequency of a sequence of on and off signals of a conventional incandescent lamp is very low because of the thermal afterglow. Because of the special matching requirements of the frequency of the high-frequency carrier, there are limitations on the oscillator and detector which may be used at any particular region or with any particular central locking system. Furthermore, at certain times, one user may elect to employ radio-frequency transmission while at other times the same user may require a light wave transmission system to operate the central locking system. Up to now, two different systems had to be provided with the transmitters and receivers being originally designed for the particular mode of wave transmission and with the transmitter and receiver including the coding and decoding units being achieved if a different mode of transmission was required.